This application claims priority from Swedish patent application 0100610-5, filed on Feb. 22, 2001, which is incorporated herein by reference.
The present invention relates to a method of improving selectivity in a tuneable waveguide filter in co-action with a laser.
Waveguide filters, also referred to as directional couplers, are formed from two adjacent waveguides and can be used as transmission filters. A problem with such filters arises when one waveguide suffers more losses than the other. In addition to a decrease in peak transmission, the transmission spectrum becomes broader and the transmission minima on either side of the transmission peak increase in transmission. This is disadvantageous, since the selectivity of the filter is reduced. In other words, the ability of the filter to transmit optical power at the desired wavelength is reduced while, at the same time, the transmission through the filter of optical power at undesired wavelengths is increased.
The shape of the transmission curve of a directional coupler is important with regard to its optical selectivity, i.e. the difference in transmission of two frequencies that are spaced a given distance apart. It may be beneficial for the transmission spectrum to fall to a zero value for frequencies on either side of the desired frequency, so that the transmission is fully extinguished by the filter with respect to certain frequencies. This is made impossible in a directional coupler when the optical absorption in the two waveguides is different.
A directional coupler is conveniently tuned by injecting charge carriers into one of the two waveguides. In addition to tuning, however, the charge carriers increase the optical losses due to free charge carrier absorption. The selectivity of the filter, therefore, is not uniform for the different frequencies over the filter""s tuning range.
This problem is addressed by the present invention, which relates to a an approach to improve the selectivity in a tunable waveguide filter, such as a layered coupling filter, that includes two waveguides, also referred to as a directional coupler filter. One of the waveguides is a semi-active waveguide. The increased absorption of charge carriers that occurs when current is injected into the semi-active waveguide is compensated by gain enhancement in the waveguide, because the material used for the semi-active waveguide is selected to provide an appropriate amount of gain.
One particular embodiment of the invention is directed to a method of improving selectivity in a tunable waveguide filter that includes first and second waveguides, the first waveguide being a semi-active waveguide. The method includes coupling light between the first and second waveguides and passing current through the first and second waveguides to tune a light frequency of maximum transmission through the filter. The light in the semi-active waveguide is amplified.
Another embodiment of the invention is directed to a tunable filter that has a first waveguide and a second waveguide proximate the first waveguide so that light couples between the first and second waveguides. The second waveguide is semi-active. A grating structure is disposed proximate at least one of the first and second waveguides to enhance coupling of light between the first and second waveguides.
The above summary of the present invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments.